Surface mounted inductors having wire wound coils in cores are widely used. Accompanied by recent miniaturization or reduction in thickness of electronic devices such as mobile phones, miniaturization or low profile is demanded also in electronic components such as surface mounted inductors. The applicant proposed, in the previously filed JP 2010-245473 A, a small surface mounted inductor using a coil obtained by spirally winding a rectangular conductive wire so that both of its ends are led out to an outer periphery and a preformed tablet, and a manufacturing method thereof.
In the manufacturing method of a surface mounted inductor described in JP 2010-245473 A, first, a sealing material containing a resin and a filler is preformed into a shape with pillar-shaped convex portions on a plate-shaped peripheral edge to prepare a tablet. Next, a coil is prepared by winding a conductive wire which is rectangular in cross section, and the coil is placed on the tablet. At this time, lead-out ends of the coil are placed so as to be along the pillar-shaped convex portions of the tablet. Then, the coil and the tablet are arranged in a molding die so that a lead-out end of the coil is interposed between an outer side surface of a pillar-shaped convex portions of the tablet and an inner wall surface of the molding die, and further the preformed sealing material is loaded into the molding die. Subsequently, the coil and the sealing material are integrated by a compression molding method or a powder compacting method in a state in which the lead-out end of the coil is interposed between the outer side surface of the pillar-shaped convex portions of the tablet and the inner wall surface of the molding die, to obtain a formed body. Lastly, as shown in FIG. 13, external electrodes 38, which are connected to portions of the lead-out ends of the coil 31 which are at least partially exposed on surfaces of a formed body 37, are provided on the surfaces or outer peripheries of the formed body.
As described above, in the method of JP 2010-245473 A, the sealing material containing the resin and the filler is preformed into the tablet with the pillar-shaped convex portions on the planar-shaped peripheral edge. In particular, considering positioning of the coil, as shown in FIG. 14, it is desired that a tablet 32 is formed into a shape with pillar-shaped convex portions 32a surrounding a coil 31. However, when attempting to obtain a surface mounted inductor having a size of two square millimeters or less, the size of the tablet also becomes small, and there is no choice but to reduce the thickness of the pillar-shaped convex portions. In the tablet formed with a sealing material containing a resin and a filler (in particular, those having a filler content of 60 vol % or more), when it is preformed into a complicated shape like having pillar-shaped convex portions, it becomes difficult to secure sufficient mechanical strength. When the mechanical strength of the tablet deteriorates, a defect or damage of a part of the tablet is liable to occur during carriage or loading into the molding die. When a defect or damage of a part of the tablet occurs, misalignment of the inside coil or forming failure occurs so that there is a possibility of causing characteristic failure or variation of inductors. Therefore, a certain degree of thickness had to be secured for the pillar-shaped convex portions of the tablet. When the thickness of the pillar-shaped convex portions of the tablet is secured in a state in which the shape is miniaturized, the coil shape becomes small, so that the characteristics of the inductor such as a DC resistance and a DC superimposed characteristic deteriorate. Therefore, there was a limit to the miniaturization or low profile in the method of JP 2010-245473 A.
In order to solve such a problem, as shown in FIG. 15, it is considered as follows: A coil 41 formed by winding a winding wire is housed in a molding die consisting of a lower die 45 and an upper die 44. Lead-out ends 41b of the coil 41 are held by the lower die and the upper die. A sealing material containing a resin and a filler is filled into the molding die so that they are pressurized using the die and a punch (for example, JP 2009-170488 A).
However, in such a case, it is required to provide a mechanism for holding the lead-out ends of the coil in the molding die and therefore the molding die becomes expensive. Also, the sealing material leaks from a portion where a lead-out end of the coil in the molding die is held, so that large burrs were likely to occur. Furthermore, when the conductive wire forming the coil is thickened, the distance between the molding die and the coil is reduced, and the filler does not penetrate resulting in the problem of deterioration of the characteristics.